1. Field of the Invention
This invention relates generally to chemical mechanical polishing apparatuses, and more particularly to methods and apparatuses for end point triggering with integrated steering in a chemical mechanical polishing system.
2. Description of the Related Art
In the prior art, chemical mechanical polishing (CMP) systems typically implement belt, orbital, or brush stations in which belts, pads, or brushes are used to scrub, buff, and polish a wafer. To facilitate and enhance the CMP operation, slurry is used. Slurry generally is introduced onto a moving preparation surface, e.g., belt, pad, brush, and the like, and distributed over the preparation surface as well as the surface of the semiconductor wafer being buffed, polished, or otherwise prepared by the CMP process. The distribution is generally accomplished by a combination of the movement of the preparation surface, the movement of the semiconductor wafer and the friction created between the semiconductor wafer and the preparation surface. In addition, end point detection mechanisms are used to determine when to end the CMP process. The end point detection mechanism senses the wafer layers through an end point window when triggered by an end point trigger mechanism.
FIG. 1A is diagram showing a prior art CMP system 100a. The CMP system 100a includes a polishing belt 101 and rollers 102, which physically rotate the belt and provide belt steering. During the CMP process, a wafer is positioned at wafer position 106, generally via a carrier having a retainer ring that holds the wafer in position during polishing. Beneath the wafer position 106 is a platen 104 for wafer support during polishing. To facilitate end point detection, an end point window 108 is disposed within the polishing belt 101.
Further disposed within the polishing belt 101 is a trigger slot 110, which is used in conjunction with an end point trigger mechanism. In operation, the end point trigger mechanism detects the trigger slot 110 whenever the trigger slot 110 passes over the end point trigger mechanism. Then, as will be described in greater detail with respect to FIG. 2, the end point trigger mechanism provides a signal to a CMP controller, which controls end point detection. In this manner, the prior art CMP system 100a can synchronize end point detection sensing with the end point window 108 in the polishing belt 101. To help maintain the centralized position of the end point window 108 with respect to the wafer position 106 and platen 104, limit sensors are used.
FIG. 1B is a diagram showing a top view of a prior art CMP system 100b. The prior art CMP system 100b includes rollers 102, and a polishing belt 101 having an end point window 108 and a trigger slot 110 to facilitate end point detection control. The end point detection mechanism performs best when the end point window 108 is centered above the end point detection sensor. Thus, limit sensors 112 are used to detect the lateral position of the polishing belt 101 during operation. When the polishing belt 101 moves off center, the limit sensors detect the position of the polishing belt 101 and provide the positional information to a belt steering mechanism. The belt steering mechanism then adjusts the lateral position of the polishing belt 101 using the rollers 102.
FIG. 2 is a diagram showing a conventional end point trigger mechanism 200. The end point trigger mechanism 200 includes a polishing belt 101 having an end point window 108 and a trigger slot 110. Further included are a trigger sensor 206, which detects light from a light source 204, and a CMP controller, which receives information from the trigger sensor 206 and an end point detection sensor 208. The end point detection sensor 208 senses the current layer status of the wafer 202 in a CMP process.
Generally, the trigger sensor 206 is an optical sensor that detects the presence of the trigger slot 110 by the intensity of the light detected from the light source 204. Specifically, the polishing belt 101 blocks light from the light source 204, except when the trigger slot is 110 present above the trigger sensor 206. Thus, when the trigger sensor 206 detects a light intensity above a predefined threshold, a message is sent to the CMP controller 212 that the trigger slot 110 is presently positioned above the trigger sensor 206. Since the trigger slot 110 is positioned a know distance from the end point window 108 and the belt speed is known, the appropriate delay can be calculated that will trigger end point data acquisition when the end point window 108 is aligned with the platen window 210 of the platen 104.
As previously mentioned, belt steering is provided using limit sensors. End point signal strength in part depends upon both the alignment of the end point window 108 above the platen window 210 in the direction transverse to belt travel and the alignment in the belt travel direction. However, conventional CMP systems 100a and 100b use two separate, unrelated methods to position the end point window 108 and synchronize the end point data acquisition. Specifically, the alignment in the direction transverse to belt travel is determined using the limit sensors, while the alignment in the belt travel direction is determined by the synchronization of the end point window 108 with the platen window 210 using the trigger sensor 206.
Using two independent methods to align the end point window 108 with the platen window 210 causes problems in both the reliability of the end point detection system and the system set up time. Thus, there is a need for reliable systems and methods for improved end point trigger mechanisms that improve end point detection reliability and reduce system setup time.
Broadly speaking, the present invention fills these needs by providing a sensor array that determines the longitudinal and transverse position of the end point window, as well as the belt speed of the polishing belt. In one embodiment, a method for end point triggering in a chemical mechanical polishing process is disclosed. A sensor array is positioned beneath a polishing belt having an end point window. The polishing belt is then rotated during the CMP process, and a transverse position of the end point window is determined based on a portion of the sensor array covered by a particular portion of the polishing belt. The particular portion of the polishing belt can be the end point window, a trigger slot, or a portion of the polishing belt covered by a reflective material. Further, the sensor array can optionally be a charged coupled device (CCD), or a linear array of sensors. In operation, the positional information is determined based on which sensors are covered by the particular portion of the polishing belt. The positional information is then communicated to a belt steering system, which corrects the transverse position of the end point window based on which sensors are covered by the particular portion of the polishing belt. The time to begin end point detection is based on a longitudinal position of the end point window, and the speed of the belt can be determined based on intensities sensed by the sensor array.
In another embodiment, an apparatus for end point triggering in a chemical mechanical polishing process is disclosed. The apparatus includes a sensor array that is disposed beneath a polishing belt, which includes an end point window. The apparatus determines a position of the end point window based on a portion of the sensor array covered by a particular portion of the polishing belt. As above, the particular portion of the polishing belt can be the end point window, a trigger slot, or a portion of the polishing belt covered by a reflective material, and the sensor array can optionally be a charged coupled device (CCD), or a linear array of sensors.
A system for end point triggering in a chemical mechanical polishing process is disclosed in a further embodiment of the present invention. The system includes a polishing belt having an end point window, and a sensor array disposed beneath a polishing belt. The sensory array can determine a position of the end point window based on a portion of the sensor array covered by a particular portion of the polishing belt, which can be can be the end point window, a trigger slot, or a portion of the polishing belt covered by a reflective material. The system further includes a belt steering system that corrects the belt position based on the position of the end point window.
Advantageously, the embodiments of the present invention logically correlate signals from multiple detector elements in an array so as to provide corroborative synchronization and steering information from multiple sensing points. This ability greatly enhances the robustness and reliability of belt steering and end point detection during the CMP process. In addition, by combining two separate functions into a single sensing apparatus, the embodiments of the present invention greatly simplify system set up and improve reliability.
Moreover, by using multiple sensors in a wet environment, the embodiments of the present invention reduce the number of errors often encountered in prior art systems, which use single sensors designed for a dry environment. Other aspects and advantages of the invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.